In recent integrated circuit chip design, it is often desired to provide passive electrical devices such as inductors directly on the chip as part of the integrated circuit. Inductors are required in various microelectronic applications, e.g. voltage control oscillators and power amplifiers.
In addition, inductors are often used in radio frequency (RF) circuits such as those used in devices like cellular telephones, wireless modems, and other types of wireless communication equipment. An inductor joined in series or parallel with a capacitor can form a frequency resonator or filter unwanted signals.
However, due to the limited space on an integrated circuit chip and highly competitive chip market, on-chip inductors must fit within a limited space and be inexpensive to fabricate. In that regard, it is desirable for an on-chip inductor to have a high inductance per unit area.
Prior art on-chip inductors with air core have encountered many problems. Typically, they require too much space on the integrated circuit chip. Often, the Q factor of the air core on-chip inductor is too low. Many prior art on-chip inductors include an air core inductor that has an open magnetic field. Such designs often generate interference and/or unwanted magnetic coupling that may cause instability problems. In addition, prior art air core on-chip inductors often exhibit an eddy current in the near metal or substrate of low volume resistivity that further reduces the inductor's Q value. Another shortcoming of prior art air core on-chip inductors is that their inductances are not adjustable. A lack of adjustability results in a low yield rate when process variations occur. Finally, prior art air core on-chip inductors are not suitable for very high frequency applications, i.e., higher than 10 GHz, because their large size presents large parasitic capacitance and the self-resonate frequency would be lower than the operation frequency.